The present invention relates to monolithically integrated control circuits for the switching of transistors, and in particular to a control circuit for the rapid switching of a power transistor which may be used to drive inductive loads in high-speed printing apparatus and, in particular, in switching supply devices, also called chopper supply devices.
The control circuits of these devices have to drive an output power transistor, connected in series with an inductive load between the two terminals of a supply voltage generator, in an alternating manner from a high voltage and low current state to a low voltage and high current state, by means of base control signals.
In its first state, the power transistor is virtually an open circuit (its cut-off or "off" condition) between its emitter and collector terminals, and in its second state the transistor is a short-circuit (its conduction or "on" condition), respectively preventing or enabling the flow of current through the inductive load. The mode of operation of a transistor which comes closest to the operation of an ideal switch is that in which the transistor operates at saturation in its closed state and is cut-off in its open state.
The maximum possible switching frequency of the output power transistor is in this case limited essentially by the effects, during the passage from saturation to turn-off, of the storage of charges in its base which took place during the conduction stage.
A circuit designed to reduce the cut-off time, so as to increase the possible switching frequency and to improve the efficiency of the control circuit from the power point of view is disclosed, for example, in the Italian patent application Ser. No. 25054 A/81.
However, when it is desired to achieve the maximum possible switching frequency while maintaining maximum efficiency, it is necessary to act on the turn-on times of the controlled transistor, in particular when it has to operate at maximum saturation in the conduction state.
It is indispensable for the controlled transistor to operate in these operating conditions when, for example in the case of switching supply devices, it is desired to reduce the values of the load inductances by increasing the switching frequency.
In this case, minimizing the collector-emitter voltage of the output transistor in series with the inductive load involves being able to supply to the load a greater portion of the available supply voltage, while meanwhile decreasing the dissipation of power in the transistor itself.
In order to obtain the maximum reduction of turn-on times of a transistor, it is necessary to supply its base with the maximum possible current during the entire switching transient from the cut-off state to the conduction state.
During this transient, the transistor operates as though its current gain were much lower than its gain during normal conduction conditions.
However, this leads to the problem of limiting the level of the current supplied as soon as the transistor starts to conduct so as to prevent the reductions in efficiency which require a driving current which is higher than the current which is strictly necessary during the entire period of conduction.
The simplest efficient circuit solution to this problem is to supply the base current to the transistor via a resistor and a capacitor which have predetermined values and which are connected in parallel.
This solution, which is conventionally used by persons skilled in the art, may only be embodied, however, by means of discrete components due to the high capacitance value required.